This invention relates to valve seals, and particularly to valve seals intended for cryogenic service.
In U.S. Pat. No. 3,642,248, issued to Robert E. Benware and owned by the assignee of the present invention, a valve sealing mechanism is disclosed in the form of a shouldered wear ring confined in an undercut circumferential groove. The wear ring has a seating portion in the form of a continuous central annular rib extending radially beyond the shouldered portion through the opening in the groove, the circumferential surface of the rib serving as a seating surface in mating contact with a complementary valve member sealing surface.
The wear ring in the Benware patent preferably is combined with an elastomeric O-type backing ring positioned between the bottom of the groove and the shouldered portion of the wear ring. The backing ring serves the dual function of urging the wear ring out of the groove into firm contact with the sealing surface and of sealing against leakage of fluid under pressure behind the wear ring.
The Benware patent also discloses alternative backing members in the form of coil garter springs for use in applications involving very high and very low temperatures which would be incompatible with the elastomeric materials of the O-type backing rings. Such coil garter springs can be used alone behind the wear ring or in combination with springable, curved stiffening plates; wedge-shaped discontinuous dummy cushions (particularly for cryogenic applications); or with a plurality of tensile hoop-like bands disposed inside the coils of the garter spring to increase preloading of the wear ring. In still another arrangement, the combination of coil garter spring and tensile bands can be replaced by a finger spring.
Although the various garter spring embodiments of the Benware patent have provided effective sealing for valves in cryogenic applications, they have inherent drawbacks. Because it is necessary to use a hard, and consequently stiff, plastic material for the wear ring when handling fluids at very low cryogenic temperatures, a very high preload is demanded from the garter spring, so that usually the above-mentioned additional tension bands are required. At the same time, the garter spring coils tend to cut into the wear ring material unless stiffening plates or dummy cushions are interposed between the coil spring and the wear ring. Assembly of such multi-component seals is difficult and time-consuming, and such assemblies have a high rejection rate under test inspections. In addition, the buildup of tolerances in a multi-component assembly requires high precision for the individual components, adding to the cost.